Urea Selective Catalytic Reaction (SCR) aftertreatment technology has been chosen industry-wide for diesel engine programs to comply with the 2010 Environmental Protection Agency (EPA)'s nitrous oxide (NOx) standards. Utilizing this technology, an aqueous urea solution is often stored onboard in a urea tank and injected via a urea injector into the vehicle exhaust, where the injected urea decomposed into ammonia (NH3) and carbon dioxide (CO2). The ammonia generated is then absorbed onto a surface of a downstream SCR catalyst, where it reacts with the NOx in the exhaust for conversion to nitrogen and water.
Urea solution is often injected into the vehicle exhaust in form of an atomized spray. Despite the use of mixing systems to keep most droplets airborne, a combination of exhaust system space constraints and engine operating conditions may cause some urea droplets to form deposits on the surface of the urea injectors and in the immediate exhaust passage. The deposit formation process can be irreversible, causing blockage of the urea injector which leads to a degradation of NOx conversion efficiency of the SCR aftertreatment system. In some cases, the urea deposits growth in the immediate exhaust passage can cause an increase in engine back pressure and a corresponding loss of the engine power.
At low and medium exhaust flows, the larger droplets of the injected urea may end up on the pipe floor surfaces, where heavy and rapid urea deposits accumulation (see, e.g., FIG. 1) takes place. At medium and high exhaust flows, the rapidly evaporating smaller drops with progressively reduced diameter may be swept towards the ceiling surfaces, where deposits formation takes place.
The use of various mixing devices has shown successful reduction of deposits formation processes for both floor and ceiling surfaces. Furthermore, the periodic regeneration (e.g., cleaning by burning accumulated soot from the surfaces) of the diesel particulate filter, occurring approximately every 10 to 30 hours of vehicle operation, results in operation of the engine so that the exhaust gas temperature is above 600° C. for up to 12 minutes. The increased exhaust gas temperature may enable removal of urea deposits from the floor and the ceiling surfaces.
However, the inventors herein also recognize that some recessed surfaces on the inside of the exhaust pipe may still be prone to deposit formation and accumulation. One common example of this phenomenon is in the dosing injector boss. An installation of a urea dosing injector, whether at the pipe bend or within a straight section, may result in incorporation of a mounting boss holding the injector in place. Facing the exhaust gas side of the pipe, the boss may have a cavity or a recess that makes the injector position somewhat remote to minimize injector exposure to high temperatures. The flow pattern of exhaust gas near the cavity may be changed such that a recirculation of gases into the cavity may take place (see, e.g., FIG. 2). The amount of recirculation may vary with operating conditions, such as exhaust flow and temperature, urea dosing rate and ambient temperature. The exhaust gas recirculation near the cavity may increase the tendency for urea deposit formation and accumulation.
As such, systems and methods for injecting liquid reductant into an engine exhaust are provided herein to address the above mentioned issues. An example system includes an injector having an outlet for injecting liquid reductant into the exhaust gas upstream of the reduction catalytic converter, a gas deflector positioned upstream of the injector where the gas deflector is configured to create a higher pressure zone upstream of the deflector and a lower pressure zone downstream of the deflector surrounding the injector outlet, a bypass flow passage configured to divert a portion of exhaust flow from the exhaust passage, the bypass flow passage having an inlet in the higher pressure zone upstream of the deflector, and a collector in fluid communication with the bypass flow passage, the collector having one or more openings for allowing the bypassed portion of exhaust to flow out of the collector openings into the exhaust gas stream to form a gas shield for the liquid reductant spray.
In this way, the gas deflector creates the higher pressure zone upstream of the deflector where the inlet of the bypass passage is located, and the lower pressure zone downstream of the deflector where the injector outlet is located. A pressure differential is thereby formed that allows a portion of the exhaust gas flow to be diverted through the bypass flow passage to form a gas shield for the liquid reductant spray. The gas shield created may also serve to decrease recirculation of exhaust gas near the injector outlet, such as in an injector boss cavity, to reduce liquid reductant deposit formation and accumulation.
In some examples, the gas deflector comprises a flange that defines a bottom surface and a pair of side walls connected to the bottom surface, where the flange is configured to direct a portion of exhaust gas towards the inlet of the bypass flow passage. Further, the one or more openings of the collector may be circumferentially located around the injector outlet, which allows the gas shield formed to be a circular gas shield surrounding the liquid reductant spray.
In some examples, the bypass flow passage is formed between a mounting flange and the injector boss. Furthermore, a channel may be machined into the mounting flange to serve as the bypass flow passage.
In another embodiment, the above issues may be at least partially addressed by a method for injecting liquid reductant into engine exhaust gas stream, comprising: injecting liquid reductant into the exhaust gas stream via an injector having, the liquid reductant injected at an injection location upstream of a reduction catalytic converter for reducing NOx components in the exhaust gas; diverting exhaust gas from upstream of the injector through a bypass flow passage; and routing the diverted exhaust gas to the injection location where the diverted exhaust gas and the liquid reductant both enter the exhaust gas stream. In this way, it is possible to utilize exhaust gas to shield the injector, thereby reducing deposits.
The inventors herein have recognized the above issues, phenomena, and potential solutions. Further, it should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.